The present invention relates to a method and apparatus for projecting a pattern from a mask onto a substrate in the lithographic production of microelectronic features and, in particular, to a method and apparatus which filters amplitude in the frequency plane of the projected image.
Because of the need to decrease size of microelectronic circuits and other features lithographically produced on semiconductor wafers, there has been considerable effort made to increase the resolution of the image projected onto the substrate from the mask containing the image pattern. Some work has been done to modify the phase of the light beam as it passes through the mask, and other work has been done to modify the amplitude of the energy beam containing the projected mask pattern image. As shown in FIG. 1A in an illustration of a typical prior art projection system, also called a stepper, an illumination source 10 projects an energy beam 12 through a mask 14 containing a pattern of the features to be formed on a wafer substrate. The projected mask pattern image 16 contains zeroth order beam 20, and first order beams 18 and 18′, as well as any other higher order beams which may have been created during mask transmission. As the projected image pattern passes through pupil plane opening 22 in a projection lens system, the different orders of energy beams are essentially unaffected so that the zeroth, first and higher orders in the projected mask pattern strike the image plane 24 on the surface of the wafer substrate. The interference of the beams then creates the desired image of the mask.
A previous attempt to modify amplitude of the zeroth diffracted order of the projected image pattern 16 and obtain a higher resolution is shown in FIG. 1B, wherein an opaque filter 26 is placed in pupil plane 22 to absorb zeroth order beam 20. This results in only the first order beams 18, 18′ and any higher order beams striking the image plane 24. As a consequence of the blocked zeroth order, the pitch of the image is doubled, as would happen with an alternating phase shifting mask. This method works well and has been confirmed in experiments before. However, introduction of the filter in tools has been hindered by the fact that an opaque filter absorbs energy and gets heated. The result of this is that the surrounding optical materials, such as glass, through which the light passes are heated to such an extent that the index of refraction is changed sufficiently to affect the imaging characteristics of the projection system, causing undesirable aberrations. Thus, there is a considerable problem in jeopardizing the optical performance of the scanning tool which projects the mask pattern image onto the wafer substrate. Accordingly, there is a need to improve resolution of the mask pattern image without affecting the optical characteristics of the scanning tool.